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Re: Web Rule Language - WRL vs SWRL

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Wed, 29 Jun 2005 22:15:28 +0100
Message-Id: <70d21a758a6560b78ddf9c9b91feef56@cs.man.ac.uk>
Cc: Bijan Parsia <bparsia@isr.umd.edu>, Jim Hendler <hendler@cs.umd.edu>, www-rdf-rules@w3.org, dreer@fh-furtwangen.de, Jos de Bruijn <jos.debruijn@deri.org>
To: Michael Kifer <kifer@cs.sunysb.edu>

On 29 Jun 2005, at 19:09, Michael Kifer wrote:

>> On 28 Jun 2005, at 20:17, Michael Kifer wrote:
>>> Ian,
>>> The difference is really between SWRL and everything else,
>> I don't understand what leads you to this bizarre conclusion! (See
>> comments bellow w.r.t. your N3 argument.)
> I don't understand your argument either.

The difference is that I *have* an argument - what you have is just a 

>>> and SWRL should
>>> be developed by a different WG, if there is a need for a rules 
>>> language
>>> sitting on top of OWL.
>> OK, so you favour option (c) - several working groups developing
>> unrelated rules languages. Interesting plan. Surely we should try to 
>> do
>> something better than that.
> Do you have a better plan (and reasons to believe that it will 
> succeed)?

Yes, I believe that I *do* have a better plan. Like Bijan, I would like 
to see "a task force, or working group, or *something*" develop a 
framework that allows for multiple language types to co-exist in the 
semantic web without giving up completely on interoperability. As has 
been discussed in various of these threads, there is already promising 
work in this direction from several groups, and so every reason to 
think that such an effort can succeed *if only we have the will to do 

>>> N3 is essentially a different syntax for F-logic and its extensions
>>> (but
>>> N3's semantics is defined by use cases ;-). As far as I can tell, 
>>> with
>>> each
>>> new presentation that I hear N3 is moving in the direction of LP.
>> I think that we should stick to discussing how things actually *are*
>> rather than directions in which you hope/believe they might be moving.
> I am discussing things as they already are. N3 now has a form of SNAF.

Maybe, but this doesn't make it LP - we have long since known how to 
support a form of SNAF in DL using the so-called K operator. N3 is also 
based on RDF, and RDF does not make a CWA and so if fundamentally 
incompatible with LP (as you seemed to agree in an answer to an earlier 
email - see below).

>> Surely N3 is *actually* a different syntax for RDF, plus some 
>> rule-like
>> extensions. As we have already agreed, LP is semantically incompatible
>> with RDF, so it does not make sense to say that N3 can be included in
>> LP.
> Did we agree about RDF?  I didn't notice that.

To quote from one of your earlier emails:

> Your argument is well-taken. However, if you are querying an ontology 
> with
> an LP language then you expect that CWA will be applied in the current
> state.
> That is, if your DLP ontology says that John has one child, Bill, and
> nothing else, then your query "get all people who have exactly one 
> child"
> (a query like the one in your paper) is expected to return John, since 
> in
> the current state he is not known to have more children.

So, an LP language would find an entailment that is *not* supported by 
RDF semantics (which would allow for models in which John had other 
children). Ergo, LP is semantically incompatible with RDF.


>> And what about the various business rules people that presented at the
>> rules workshop? Are they also "moving in the direction of LP"? Didn't
>> look like it from where I was sitting!
> There was a number of different presentations. Mostly usecases.
> Most people didn't address the foundations at all.
> 	--michael
>> Ian
>>> 	--michael
>>> Ian Horrocks wrote:
>>>> On 23 Jun 2005, at 05:54, Michael Kifer wrote:
>>>>> Jim,
>>>>> So, you are saying that LP is at the same stage as DAML+OIL before
>>>>> standardization. What is needed is to work out the details --
>>>>> typically a
>>>>> job for a working group.  Who disagrees with that?
>>>> I do not agree that the current situation w.r.t. "rules languages" 
>>>> is
>>>> comparable to the one appertaining w.r.t. "ontology languages" when
>>>> the
>>>> WebOnt working group was chartered. At that time there was a single
>>>> candidate ontology language around which a broad consensus had 
>>>> already
>>>> been built (e.g., through the merging of the OIL and DAML-Ont
>>>> efforts).
>>>> Currently there are several competing rules language proposals, with
>>>> no
>>>> obvious (to me) leading contender.
>>>> I also disagree with the suggestion that a Working Group is likely 
>>>> to
>>>> be able to resolve major technical problems - if you look at what 
>>>> went
>>>> in to WebOnt (DAML+OIL) and what came out (OWL), you will see that
>>>> they
>>>> are relatively similar. This is not to minimise the quality or
>>>> quantity
>>>> of the work carried out within WebOnt - it simply illustrates how
>>>> difficult it is to get a large and heterogeneous WG to agree on
>>>> anything, never mind agreeing on significant technical changes, and
>>>> how
>>>> much effort is required to go from a prototype to a finished 
>>>> product.
>>>> So, your argument leads me to the conclusion that either (a) one of
>>>> the
>>>> candidates should be (arbitrarily?) chosen for standardisation, (b) 
>>>> a
>>>> WG should be established without any clear indication as to what
>>>> should
>>>> be standardised, or (c) several WG's should be established - one for
>>>> LP, one SWRL, one for N3, etc. Option (a) is hardly likely to 
>>>> promote
>>>> consensus building, option (b) seems to be a recipe for years of
>>>> unproductive argument, and option (c) would be very costly (for both
>>>> the W3C and the semantic web community), and very confusing for 
>>>> those
>>>> considering the adoption of semantic web technology.
>>>> A fourth alternative, and one that I think several people in this
>>>> thread have been arguing for, is to continue working (in whatever
>>>> context) towards an architectural framework that provides for a 
>>>> better
>>>> integration between First Order and LP based languages. We should 
>>>> then
>>>> be able to achieve the broad consensus which is, I believe, a
>>>> necessary
>>>> (or at least highly desirable) precursor to the initiation of a
>>>> standardisation activity.
>>>> Regards,
>>>> Ian
>>>>> And you didn't need to defend OWL because nobody was attacking it.
>>>>> Our
>>>>> discussion was about the 1-stack vs. multi-stack architectures. 
>>>>> Since
>>>>> in a
>>>>> previous email you said that you are "not against a multi-stack
>>>>> solution"
>>>>> then I don't see what is the point of contention (at least at the
>>>>> high
>>>>> level). I believe that everybody agrees that the more integration 
>>>>> --
>>>>> the
>>>>> better. We just don't believe that integration of the leading 
>>>>> useful
>>>>> technologies to the point that only one stack is left is possible.
>>>>> 	--michael
>>>>>> Mike - I think you're missing the point of the "webbie" nature of
>>>>>> OWL
>>>>>> and the difference from traditional KR, but I have written that up
>>>>>> too many times to do it again here.  With LP, the question is how 
>>>>>> I
>>>>>> can use your rules/program/etc. in part to get a "network effect"
>>>>>> and
>>>>>> to make it so I can link together the logics and logic programs as
>>>>>> easily as I link web pages.  It;s not that no one has good 
>>>>>> research
>>>>>> ideas on how to do that, it's how to bring those to fruition and
>>>>>> greater use that is the key
>>>>>>     The ontology stuff in OWL, which is not actually DL (even  OWL
>>>>>> DL
>>>>>> departs from traditional DL in some interesting ways, but OWL Full
>>>>>> is
>>>>>> the one I care most about) is based on many years of work in AI, 
>>>>>> and
>>>>>> was explored on the web long before OWL was done - cf the SHOE 
>>>>>> work
>>>>>> my group did (still a high hit at Google - so just google "shoe")
>>>>>> and
>>>>>> that was followed by XOL, OIL, and others before the 
>>>>>> standardization
>>>>>> began.
>>>>>>   I think the LP stuff is in similar state - a basic idea has been
>>>>>> fleshed out, some variants are being explored, and there is a govt
>>>>>> interest in pushing for a de facto standard.   But  going from 
>>>>>> there
>>>>>> to the finish line is where a lot of the time and blood goes in --
>>>>>> it's in making the stuff fit with what else is out there in the 
>>>>>> Web.
>>>>>> We had to do a lot of work to make OWL fit in with RDF and other
>>>>>> languages it needed to interoperate with, and a web rules language
>>>>>> needs to be defined with the other things already in the space it
>>>>>> wants to play in (thus the "stacks" issue - if it wants to be in a
>>>>>> Sem Web stack, it needs to play with other SW stuff; if it wants 
>>>>>> to
>>>>>> be in the XML stack, it needs to play nice with XML stuff like
>>>>>> Xquery
>>>>>> and Xpath, etc.
>>>>>>   And that is the discussion we are having -- but if we can nail
>>>>>> this
>>>>>> stuff, the result is worth it -- OWL is certainly the most used
>>>>>> KR/ontology langauge in the history of AI as best anyone can tell,
>>>>>> and if we want the Web Rules Language to flourish we want it to 
>>>>>> grow
>>>>>> like the Web does, not like rules languages have -- nothing wrong
>>>>>> with the latter, but there's a whole lot more Web pages out there
>>>>>> than logic programs, and it's a lot more fun to play in the
>>>>>> exponential growth space :-)
>>>>>>   anyway, we're all working for same ends, just different means, 
>>>>>> and
>>>>>> finding the consensus space in the middle is wondefully non-fun, 
>>>>>> but
>>>>>> worth it in the end
>>>>>>   Ok, end of crap, back to technical issues and Greek symbols...
>>>>>>   JH
>>>>>> p.s. please note - I spent many years of my career arguing against
>>>>>> DL
>>>>>> and doing scruffy AI - yet here I am defending OWL - why?  because
>>>>>> the design time and fights over the details of a number of use 
>>>>>> cases
>>>>>> ended up creating something pretty damn useful -- both in the OWL 
>>>>>> DL
>>>>>> space and in the OWL Full space -- so somehow the process 
>>>>>> worked...
>>>>>> At 20:41 -0400 6/22/05, Michael Kifer wrote:
>>>>>>> Jim Hendler wrote:
>>>>>>>>  Mike - I think you misunderstand the stuff about stacks and etc
>>>>>>>> --
>>>>>>>> I
>>>>>>>>  hope my use cases (in public-sws-ig@w3.org for those just 
>>>>>>>> joining
>>>>>>>> the
>>>>>>>>  conversation) would help make it clear that these are not
>>>>>>>> separate
>>>>>>>>  and unrelated stacks, nor are they identical things -- the key 
>>>>>>>> is
>>>>>>>>  figuring out how the stacking works and how things interact --
>>>>>>>> I'm
>>>>>>>>  not against a "multi-stack: solution, but as far as I am
>>>>>>>> concerned
>>>>>>>>  the more overlap the better, and I am fairly sure that we can 
>>>>>>>> do
>>>>>>>>  significantly better than DLP in terms of providing a useful 
>>>>>>>> web
>>>>>>>>  rules language that interacts well with the existing, and
>>>>>>>> becoming
>>>>>>>>  more widely used, ontology spec.*
>>>>>>> I think it is not just me, but a number of people who read your
>>>>>>> paper on
>>>>>>> the two stacks may have misunderstood it. At least one way to
>>>>>>> understand
>>>>>>> what is said there is that 1 stack is good and 2 is not.
>>>>>>> If there is another way to understand it (as advocating a
>>>>>>> multi-stack
>>>>>>> architecture) then this second meaning is deeper than I was able 
>>>>>>> to
>>>>>>> dig up.
>>>>>>>>    Seems to me the key is exploring how to get maximum
>>>>>>>> interoperability
>>>>>>>>  between the important work in BOTH areas (and I defy you to go
>>>>>>>> back
>>>>>>>>  through this discussion and find any email where I haven't said
>>>>>>>> I'm
>>>>>>>>  in favor of a web rules language)
>>>>>>> You didn't say this and I didn't say that you said this.  I was
>>>>>>> focusing on what I think were technically inaccurate claims in 
>>>>>>> your
>>>>>>> email
>>>>>>> regarding the layering of WRL on top of DLP (where WRL is taken 
>>>>>>> to
>>>>>>> mean the
>>>>>>> particular language under this name and not "a" generic web rules
>>>>>>> language).
>>>>>>>>  and also how to get the Web rules
>>>>>>>>  to join in the growing whole that is the semantic web -- it's 
>>>>>>>> not
>>>>>>>> the
>>>>>>>>  same as applying LP in the Web area -- I argued for nearly a
>>>>>>>> decade
>>>>>>>>  about the difference between Web Ontology and standard AI KR
>>>>>>>>  languages, and OWL has some significant differences from
>>>>>>>> traditional
>>>>>>>>  AI (see the OWL FAQ [1] and the discussion of KR  back in the
>>>>>>>> 2001
>>>>>>>>  Scientific American article [2])
>>>>>>> Not "applying LP in the Web area" but "adapting LP to the Web".
>>>>>>> Technically, OWL is an adaptation of DL to the Web with some
>>>>>>> additional
>>>>>>> research needed to accommodate RDFS.  But in the LP area this
>>>>>>> research has
>>>>>>> already been done years ago.
>>>>>>>>  This latter, btw, explains why URIs
>>>>>>>>  are not just some wildassed thing, they're crucial to the
>>>>>>>> Semantic
>>>>>>>>  Web in a very deep way - read the Sci Am or any of Tim's
>>>>>>>> discussions
>>>>>>>>  of this issue.
>>>>>>> Of course URIs are crucial. After all, they are object 
>>>>>>> identifiers,
>>>>>>> so they
>>>>>>> are as crucial as any notion of an Id.
>>>>>>> But do they imply/require a new kind of KR?  There are 
>>>>>>> interesting
>>>>>>> new
>>>>>>> problems that stem from the architecture, but don't make it sound
>>>>>>> as
>>>>>>> if the
>>>>>>> "old KR" is out of the window and adapting it to the new
>>>>>>> architecture is a
>>>>>>> hard or pointless exercise. The LP paradigm is as applicable to 
>>>>>>> the
>>>>>>> Web as
>>>>>>> DL, if not more. (I, of course, think that it is more :-)
>>>>>>>>    so, I don't see this as in any way being a discussion of 
>>>>>>>> rules
>>>>>>>> vs.
>>>>>>>>  ontology -- in fact, I cannot think of any dumber way to 
>>>>>>>> approach
>>>>>>>> it
>>>>>>>>  -- rather it seems to me we're trying to explore where these
>>>>>>>> things
>>>>>>>>  can overlap to the benefit of users and of the Web -- that
>>>>>>>> strikes
>>>>>>>> me
>>>>>>>>  as a very worthwhile pursuit
>>>>>>> The term "rules" is ambiguous in the context of our discussion. 
>>>>>>> If
>>>>>>> you said
>>>>>>> "I don't see this as in any way being a discussion of *LP* vs.
>>>>>>> ontology"
>>>>>>> then this is exactly what I was trying to say. As I remarked 
>>>>>>> above,
>>>>>>> the 2tower paper **appears** to be arguing that LP+OWL in a 
>>>>>>> 2-stack
>>>>>>> architecture is a nonstarter.
>>>>>>> 	--michael
>>>>>>>>    -JH
>>>>>>>>  [1] http://www.w3.org/2003/08/owlfaq.html
>>>>>>>>  [2]
>>>>>>>> http://www.sciam.com/article.cfm?articleID=00048144-10D2-1C70
>>>>>>>> -84A9809EC588EF21
>>>>>> -- 
>>>>>> Professor James Hendler			  Director
>>>>>> Joint Institute for Knowledge Discovery	  	  301-405-2696
>>>>>> UMIACS, Univ of Maryland			  301-314-9734 (Fax)
>>>>>> College Park, MD 20742	 		  http://www.cs.umd.edu/users/hendler
Received on Wednesday, 29 June 2005 21:15:39 UTC

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